Recirculating hot water systems are frequently used in hotels, motels and other large buildings where many hot water faucets or outlet taps are used at various locations throughout the buildings. In such systems, the hot water is supplied from the hot water tank or reservoir through a supply pipe to the hot water faucets and then is returned by way of a return pipe through a pump back to the hot water reservoir. The pump is operated to circulate water from the top or hot water side of the storage reservoir back to the bottom, so that hot water is continuously available throughout the length of the supply pipe. The result is the availability of "instant" hot water at any one of the faucets or taps throughout the system.
An alternative system for providing the availability of nearly instantaneous hot water at the hot water taps is to use a gravity system instead of a recirculating electrically controlled pump. In a gravity system, each hot water tap or faucet which is successively farther from the hot water reservoir is supplied from a supply line that is lower in height from the preceding faucet or tap in the system. The supply line from the last hot water tap in the system then is connected to a return line to supply the cooler water back to the hot water heater or reservoir. Natural gravity circulation takes place in such a system, because hot water rises and cold water sinks. An advantage of gravity systems is that the natural gravity circulation also is self-limiting after the entire line heats up.
Each of the recirculating hot water systems which have been described above and which are available in the prior art saves water which otherwise would be wasted by the user running hot water from a hot water faucet until the water leaving the faucet becomes hot. As a consequence, such systems are especially attractive in areas where water supplies must be carefully conserved. A disadvantage of recirculating hot water systems, however, is that even if the water supply pipes are well insulated, there is a substantial waste of energy to maintain the water throughout the system hot at all times. Because of the significantly increasing costs of energy over the past few years, the advantages of water savings in recirculating hot water systems are substantially offset by the energy wasted in maintaining the water throughout the system hot at all times. Furthermore, this wasted energy results in considerably increased costs for recirculating hot water systems, as compared with nonrecirculating systems; so that such recirculating systems have generally been limited to commercial uses, such as hotels and motels and have found only limited use in homes and individual residences.
In an effort to reduce the waste of energy and expense of operating recirculating hot water systems, heat sensing thermostats have been used to sense the water temperature in the return pipe and to control the turning on and off of the recirculating pump motor in accordance with the temperature sensed in the line. While this reduces the cost of the system somewhat, the inherent nature of such thermostatically controlled systems is that the temperature of the water in the pipes still is maintained at a fairly high level, so that the reduction in energy waste is not particularly significant. Such a system is disclosed in the prior art patent to Laube et al., U.S. Pat. No. 3,383,495, issued May 14, 1968.
A system which limits the operation of the recirculating pump of a recirculating hot water system to those times when instant hot water demands are most likely to occur is disclosed in the patent to Durdin, U.S. Pat. No. 1,780,379, issued Nov. 4, 1930. The Durdin system employs a timer to override the thermostatically controlled operation of the pump and prevent its operation during certain times of the day, such as during the night when there is little likelihood of demand for hot water in the system.
The Durdin system, however, has a gravity bypass in it; so that even though the electrically operated recirculating pump may be prevented from operation at certain times of the day, the recirculating system still operates under the gravity bypass principle. While some advantages may be present in the Durdin system, it clearly arbitrarily limits the operation of the pump in the system to the preset control of the timer. If this preset control does not coincide with the times of usage of the persons desiring to draw hot water from the system, the disadvantages of hot water systems which are not recirculating (that is relatively cold water in the line) may be present as if the recirculating system were not even used. In addition, if the gravity bypass part of the Durdin system is correctly installed, there would appear to be little need for the recirculating pump (except that it does speed up the distribution of hot water throughout the system); so that the wasted energy which occurs from constantly maintaining the line hot still is present in Durdin.
Another disadvantage of clock-controlled or time-controlled systems for operating the pump is that if a power failure should occur, the settings of the timer have to be readjusted or the system supplies hot water in the line at times other than the times intended.
Accordingly, it is desirable to provide a recirculating hot water system which has the advantages of providing nearly instantaneous hot water availability at the hot water taps or faucets when needed and yet which is more conservative of energy consumption than previous recirculating hot water systems. In addition, it is desirable to provide a recirculating hot water system which is simple to operate and simple to install.